The invention relates to the general field of magnetic disk systems with particular reference to very thin GMR based read heads
Read-write heads for magnetic disk systems have undergone substantial development during the last few years. In particular, older systems in which a single device was used for both reading and writing, have given way to configurations in which the two functions are performed by different structures. An example of such a read-write head is schematically illustrated in FIG. 1. The magnetic field that xe2x80x98writesxe2x80x99 a bit at the surface of recording medium 15 is generated by a flat coil, two of whose windings 14 can be seen in the figure. The magnetic flux generated by the flat coil is concentrated within pole pieces 12 and 13 which, while being connected at a point beyond the top edge of the figure, are separated by small gap 16. Thus, most of the magnetic flux generated by the flat coil passes across this gap with fringing fields extending out for a short distance where the field is still powerful enough to magnetize a small portion of recoding medium 15.
The present invention is directed towards the design of read element 20 which can be seen to be a thin slice of material located between magnetic shields 11 and 12 (12 doing double duty as a pole piece, as just discussed). The principle governing the operation of read sensor 20 is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). Most magnetic materials exhibit anisotropic behavior in that they have a preferred direction along which they are most easily magnetized (known as the easy axis). The magneto-resistance effect manifests itself as a decrease in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said decrease being reduced to zero when magnetization is along the easy axis. Thus, any magnetic field that changes the direction of magnetization in a magneto-resistive material can be detected as a change in resistance.
It is widely known that the magneto-resistance effect can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of a spin valve structure are shown in FIG. 2. In addition to a seed layer 22 on a substrate 21 and a topmost cap layer 27, these key elements are two magnetic layers 23 and 25, separated by a non-magnetic layer 24. The thickness of layer 24 is chosen so that layers 23 and 25 are sufficiently far apart for exchange effects to be negligible (the layers do not influence each other""s magnetic behavior at the atomic level) but are close enough to be within the mean free path of conduction electrons in the material. If, now, layers 23 and 25 are magnetized in opposite directions and a current is passed though them along the direction of magnetization (such as direction 28 in the figure), half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing over from 23 to 25 (or vice versa). However, once these electrons xe2x80x98switch sidesxe2x80x99, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure.
In order to make use of the GMR effect, the direction of magnetization of one the layers 23 and 25 is permanently fixed, or pinned. In FIG. 2 it is layer 25 that is pinned. Pinning is achieved by first magnetizing the layer (by depositing and/or annealing it in the presence of a magnetic field) and then permanently maintaining the magnetization by over coating with a layer of antiferromagnetic material, or AFM, (layer 26 in the figure). Layer 23, by contrast, is a xe2x80x9cfree layerxe2x80x9d whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface 15 of a magnetic disk).
The structure shown in FIG. 2 is referred to as a top spin valve because the pinned layer is at the top. It is also possible to form a xe2x80x98bottom spin valvexe2x80x99 structure where the pinned layer is deposited first (immediately after the seed and pinning layers). In that case the cap layer would, of course, be over the free layer.
A routine search of the prior art was conducted. While several references to various laminated structures within spin valves were encountered, none of these teach how the GMR ratio may be maintained at a high value while at the same time reducing the overall thickness of the structure. Several of the references found were, however, of interest. For example, Fontana, Jr. et al. (U.S. Pat. No. 5,701,223) forms a pinned layer by strongly coupling two ferromagnetic films in an antiferromagnetic configuration i.e. the two films have a relative antiparallel orientation. The magnetic moments of the two ferromagnetic layers are required to be almost the same. Under these conditions, a pinning layer of nickel oxide may be used and this has the advantage that a cap layer is no longer needed.
In U.S. Pat. No. 5,920,446, Gill describes a laminated free layer formed from two ferromagnetic layers separated by a non-magnetic, conducting spacer layer. A key feature is that the two outer layers of the laminate are coupled in an anti-parallel configuration. This arrangement allows the device to operate without a pinned (or pinning) layer.
In U.S. Pat. No. 5,790,351, Suzuki describes a magnetoresistive head in which longitudinal bias is supplied through a film positioned to be on either side of the magnetoresistive element, this sub-structure lying on a ground plane of copper.
It has been an object of the present invention to provide a spin valve structure that is both thinner than prior art structures as well as having a good GMR ratio.
Another object of the invention has been to enhance the magnetic-anisotropy of a GMR free layer made up of only CoFe or COFeX.
A further object of the invention has been to provide a process for manufacturing said spin valve structure.
These objects have been achieved by inserting a seed enhancement layer between the seed and the free layer. This seed enhancement layer must have a FCC crystal structure. Our preferred material for it has been NiCu. When this layer is present, a constant GMR ratio of about 7% is obtained for a thickness range of the free layer of from 10 to 40 angstroms. A process for manufacturing this structure is also disclosed.